Processes to avoid anodic oxide delamination of anodized high strength aluminum alloys

ABSTRACT

Methods of forming anodic oxide coatings on certain high strength aluminum alloys are described. Methods involve preventing or reducing the formation of interface-weakening species, such as zinc-sulfur compounds, at an interface between an anodic oxide coating and underlying aluminum alloy substrate during anodizing. In some embodiments, a micro-alloying element is added in very small amounts to an aluminum alloy substrate to prevent enrichment of zinc at the anodic oxide and substrate interface, thereby reducing or preventing formation of the zinc-sulfur interface-weakening species. In some embodiments, a sulfur-scavenging species is added to an aluminum alloy substrate to prevent sulfur from a sulfuric acid anodizing bath from binding with zinc and forming the zinc-sulfur interface-weakening species at the anodic oxide and substrate interface. In some embodiments, a micro-alloying element and a sulfur-scavenging species are added to an aluminum alloy substrate. Resultant anodic oxide coatings have minimal or no discoloration.

FIELD

This disclosure relates generally to anodic oxide coatings and methodsfor forming the same. In particular, methods for preventing formation ofcompounds during anodizing of certain high-strength aluminum alloysubstrates that can weaken the interfacial adhesion of a resultantanodic oxide coating are described.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 14/474,021,entitled “PROCESS TO MITIGATE SPALLATION OF ANODIC OXIDE COATINGS FROMHIGH STRENGTH SUBSTRATE ALLOYS,” filed on Aug. 29, 2014; U.S.application Ser. No. 14/593,845, entitled “PROCESSES TO REDUCEINTERFACIAL ENRICHMENT OF ALLOYING ELEMENTS UNDER ANODIC OXIDE FILMS ANDIMPROVE ANODIZED APPEARANCE OF HEAT TREATABLE ALLOYS,” filed on Jan. 9,2015; U.S. application Ser. No. 14/678,881, entitled “PROCESS FOREVALUATION OF DELAMINATION-RESISTANCE OF HARD COATINGS ON METALSUBSTRATES,” filed on Apr. 3, 2015; and U.S. application Ser. No.14/678,868, entitled “PROCESS TO MITIGATE GRAIN TEXTURE DIFFERENTIALGROWTH RATES IN MIRROR-FINISH ANODIZED ALUMINIUM,” filed on Apr. 3,2015, each of which is incorporated herein in its entirety.

Any publications, patents, and patent applications referred to in theinstant specification are herein incorporated by reference in theirentireties. To the extent that the publications, patents, or patentapplications incorporated by reference contradict the disclosurecontained in the instant specification, the instant specification isintended to supersede and/or take precedence over any such contradictorymaterial.

BACKGROUND

Anodizing of aluminum is most commonly performed in sulfuric-acid basedsolutions, for example, using the processes defined as “Type II” and“Type III” by MIL-A-8625. The resultant anodic oxide coatings providegood wear and corrosion resistance to the substrate, and Type IIcoatings in particular, have a good cosmetic appearance. On certainalloys, and within certain process constraints, the resulting oxidelayer may be clear and substantially colorless, giving a bright metallicappearance which is a highly desirable finish for the aluminum housingof consumer electronic devices. The anodic oxides are also conducive totaking on dyes for coloring. Thus, type II and III anodizing processesare widely used in various industries.

During type II and III anodizing, sulfur-based anions from the sulfuricacid solution become incorporated within the resulting anodic oxidecoating. These sulfur-based anions can combine with certain alloyingelements originating from aluminum alloy substrates and that accumulateat an interface between the anodic oxide coating and the aluminum alloysubstrate. For example, zinc is a common alloying element found in manyhigh-strength aluminum alloys, notably the 7000-series, of which it isthe defining alloying element (as per the International AlloyDesignation System). Zinc is less readily oxidized than aluminum, andtherefore accumulates at the interface between the anodic oxide coatingand aluminum alloy substrate. When the sulfur-based anions combine withzinc enriched at the interface, zinc-sulfur compounds form at theinterface. It has been found that these zinc-sulfur compounds can weakenadhesion of the anodic oxide coating to the substrate and cause theanodic oxide coating to be susceptible to delamination (i.e., chippingor peeling), particularly in alloys designed to satisfy both a highstrength requirement, and anodizing cosmetics.

SUMMARY

This paper describes various embodiments that relate to anodizingprocesses and anodic oxide coatings using the same. The methodsdescribed are used to form anodic oxide coatings with strong interfacialadhesion by avoiding formation of interface-weakening species at aninterface between the anodic oxide coatings and underlying substratesduring anodizing of aluminum alloy substrates.

According to one embodiment, an enclosure for an electronic device isdescribed. The enclosure includes an aluminum alloy substrate includingzinc, magnesium, and a micro-alloying element. A concentration of themicro-alloying element is at most 0.1 weight %. The enclosure alsoincludes an anodic oxide formed on the aluminum alloy substrate. Themicro-alloying element is enriched at an interface between the aluminumalloy substrate and the anodic oxide.

According to another embodiment, a method of forming an enclosure for anelectronic device is described. The method includes anodizing analuminum alloy substrate that includes zinc, magnesium, and amicro-alloying element. A concentration of the micro-alloying elementwithin the aluminum alloy substrate is at most 0.1 weight %. Themicro-alloying element reduces enrichment of the zinc at an interfacebetween the aluminum alloy substrate and a resultant anodic oxide.Enrichment of the zinc at the interface is associated with reducing anadhesion of the anodic oxide to the aluminum alloy substrate.

According to a further embodiment, a method of forming an enclosure foran electronic device is described. The method includes anodizing analuminum alloy substrate that includes zinc, magnesium, and amicro-alloying element. A concentration of the micro-alloying elementwithin the aluminum alloy substrate is at most 0.1 weight %. Themicro-alloying element reduces the discrepancy between the anodic oxidegrowth rates on grains having surface orientations of {111} and those ofother orientations. Grain structures having {111} orientation associatedwith preferential anodic oxide growth and defects within the anodizedaluminum alloy substrate.

According to an additional embodiment, a part is described. The partincludes an aluminum alloy substrate including zinc as an alloyingelement. The part also includes an anodic oxide coating formed on thealuminum alloy substrate, the anodic oxide coating including a sulfurspecies incorporated therein, wherein the anodized part is characterizedas having a CIELAB b* color space parameter value between −1 and 1.

According to another embodiment, a method of anodizing an aluminum alloysubstrate comprising zinc is described. The method includes anodizingthe aluminum alloy substrate in a sulfuric acid-based solution. A sulfurspecies from the sulfuric acid-based solution becomes incorporatedwithin a resultant anodic oxide coating. Some of the zinc becomesenriched at an interface between the anodic oxide coating and aluminumalloy substrate during the anodizing. The aluminum alloy substrateincludes a sulfur-scavenging species that binds with the sulfur speciespreventing at least some of the enriched zinc from forming a zinc-sulfurcompound at the interface. The zinc-sulfur compound is to be avoided orminimized because it is reduces the interfacial adhesion between theanodic oxide coating and the aluminum alloy substrate.

According to a further embodiment, an enclosure for an electronic deviceis described. The enclosure includes an aluminum alloy substrateincluding zinc and magnesium. The enclosure also includes an anodicoxide coating formed on the aluminum alloy substrate. The anodic oxidecoating includes a sulfur species incorporated therein. Some of thesulfur species is bonded with a sulfur-scavenging species that preventsthe sulfur species from binding with the zinc. The magnesium may itselfact as the sulfur-scavenging species if it is present in a substantialexcess over the balanced level required for the formation ofzinc-magnesium precipitates to give a certain target strength orhardness in the alloy.

These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements.

FIGS. 1A and 1B show schematic cross-sections of a surface portion of apart, showing how sulfur-based anodic bath anodizing of zinc-containingaluminum alloys can form interface-weakening species.

FIG. 1C shows an EELS graph and a high-resolution microscopic imageindicating evidence of interfacial zinc enrichment for an anodizedzinc-containing aluminum alloy substrate.

FIG. 2 shows a schematic cross-section of a surface portion of a partformed using a substrate having a micro-alloying element that preventsor reduces formation of interface-weakening species.

FIG. 3 shows a graph indicating interfacial enrichment for a number ofelements as a function of Gibbs free energy.

FIG. 4 shows an EELS graph and a high-resolution microscopic imageindicating evidence of prevention of interfacial zinc enrichment whenusing copper as a micro-alloying element.

FIG. 5A shows a graph indicating yellowing effects on anodic films ofaluminum alloy substrates with different amounts of copper.

FIG. 5B shows a graph indicating anodic oxide grown uniformity anddefect reduction by using copper as a micro-alloying element.

FIG. 6A shows a flowchart illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate usinga micro-alloying element.

FIG. 6B shows a flowchart illustrating a process of reducinggrain-related defects in an anodized high-strength substrate using amicro-alloying element.

FIG. 7 shows a schematic cross-section of a surface portion of a partformed using a substrate having a sulfur-scavenging species thatprevents or reduces formation of interface-weakening species.

FIG. 8 shows an annotated periodic table summarizing some criterion forchoosing a suitable sulfur-scavenging species in accordance with someembodiments.

FIG. 9 shows a graph indicating magnesium and zinc concentrations ofdifferent commercially available 7000 series aluminum alloys and customalloy compositions.

FIG. 10 shows a flowchart illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate usinga sulfur-scavenging species.

FIG. 11 shows a flowchart illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate usinga combination of sulfur-scavenging species and micro-alloying element.

FIG. 12 shows a graph indicating anodic oxide adhesion improvement byusing copper as a micro-alloying element and lithium as asulfur-scavenging species.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodimentsillustrated in the accompanying drawings. It should be understood thatthe following descriptions are not intended to limit the embodiments toone preferred embodiment. To the contrary, they are intended to coveralternatives, modifications, and equivalents as can be included withinthe spirit and scope of the described embodiments.

Described herein are processes for increasing the adhesion strength ofanodic oxide coatings on certain high-strength aluminum alloysubstrates. Methods involve preventing the formation ofinterface-weakening species from forming at an interface between ananodic oxide coating and underlying aluminum alloy metal base. Aninterface-weakening species is an element or compound that resides atthis interface and weakens the bond strength between the anodic oxidecoating and metal base, thereby rendering the anodic oxide coatingsusceptible to chipping, peeling, or spalling. A particular type ofinterface-weakening species is zinc-sulfur species, such as a zincsulfate or a zinc sulfite. The zinc originates from the aluminum alloyas an alloying element, and the sulfur originates from asulfur-containing anodizing solution (e.g., sulfuric acid-basedsolution). A number of other aluminum alloying elements other thanzinc-sulfur species have also been shown to form interface-weakeningspecies at the substrate and anodic oxide coating interface.

Methods described herein involve adding one or more elements to thealuminum alloy substrate prior to anodizing so as to prevent or reducethe formation of interface-weakening species at the substrate and anodicoxide coating interface. In some embodiments, the one or more elementsenrich at the interface more favorably than the interface-weakeningspecies, which prevents or reduces the enrichment of interface-weakeningspecies at the interface. In some embodiments, the one or more elementsbind with sulfur originating from an anodizing solution duringanodizing. This prevents or reduces the occurrence of zinc and/or otherelements associated with delamination from combining with the sulfur toform interface-weakening species at the interface.

The present paper makes specific reference to aluminum alloys andaluminum oxide coatings, and particularly to 7000-series alloys ofaluminum, which comprise zinc-based strengthening precipitates. Itshould be understood, however, that the methods described herein may beapplicable to other types of aluminum alloys—such as 8000-series, whichcontain lithium and zinc alloying elements—and possibly also to any of anumber of other suitable anodizable metal alloys, such as suitablealloys of titanium, zinc, magnesium, niobium, zirconium, hathium, andtantalum, or suitable combinations thereof. As used herein, the termsanodic oxide, anodic oxide coating, anodic film, anodic layer, anodiccoating, oxide film, oxide layer, oxide coating, etc. can be usedinterchangeably and can refer to suitable metal oxide materials, unlessotherwise specified.

Methods described herein are well suited for providing cosmeticallyappealing surface finishes to consumer products. For example, themethods described herein can be used to form durable and cosmeticallyappealing anodized finishes for housing for computers, portableelectronic devices, wearable electronic devices, and electronic deviceaccessories, such as those manufactured by Apple Inc., based inCupertino, Calif.

These and other embodiments are discussed below with reference to FIGS.1A-12. However, those skilled in the art will readily appreciate thatthe detailed description given herein with respect to these Figures isfor explanatory purposes only and should not be construed as limiting.

FIGS. 1A and 1B show schematic cross-section views of a surface portionof part 100, showing how sulfur-based anodizing (e.g., Type IIanodizing) of zinc-containing aluminum alloys can form zinc-sulfurinterface-weakening species. Part 100 includes aluminum alloy substrate102, a portion of which has been converted to anodic oxide 104, whichincludes anodic pores 110 that are formed during the anodizing process.The region between anodic oxide 104 and substrate 102 can be referred toas interface 108. Substrate 102 includes aluminum matrix 112 and zinc106, which serves as an alloying element in many aluminum alloycompositions to increase the strength and hardness of the aluminumalloy, with 7000-series aluminum alloys (per The International AlloyDesignation System) generally having relatively high levels of zinc 106.In some applications, it is necessary for substrate 102 in a T6 temperto have a yield strength of at least 330 MPa. In some embodiments, thiscorresponds to a zinc concentration of at least 4 weight %, in somecases as little as 2 weight %. Note that zinc is schematically shown aspoints 106 in FIGS. 1A and 1B, though it may be either uniformlydistributed through aluminum matrix 112, or concentrated within discreteprecipitates, or both. Zinc 106 can combine with magnesium (not shown)as another alloying element to form precipitates such as MgZn₂ (the η′or “eta-prime” phase), which gives substrate 102 its high strength. Theatomic % ratio of magnesium to zinc is thus optimally about 1:2. Forsimplicity, magnesium is not shown in FIGS. 1A and 1B. In addition, mostcommercially available aluminum alloys contain other alloying elementsthat are not shown in FIGS. 1A and 1B for simplicity.

During the anodizing process, alloying elements from substrate 102behave in various ways according to their relative Gibbs free energiesfor oxide formation. For example, elements that are more readilyoxidized than aluminum matrix 112, such as lithium, magnesium, calcium,scandium, yttrium, and lanthanum, are generally anodized along withaluminum matrix 112. They become incorporated into anodic oxide 104 andcan then migrate through interface 108 between substrate 102 anodicoxide 104 at rates that depend primarily on the relative mobilities ofions of these elements through anodic oxide 104, which is, in part,determined by the relative strengths of bonding between these ions andoxygen ions within anodic oxide 104. Some ions, including those oflithium, magnesium, and yttrium, migrate through anodic oxide 104 at afaster rate than aluminum ions.

Conversely, alloying elements that are less readily oxidized thanaluminum—that is, elements with more positive Gibbs free energies foroxide formation than that of aluminum—tend to become enriched atinterface 108. Examples of elements that can become enriched atinterface 108 include copper, zinc, nickel, tin, silver, and gold. Morediscussion with regard to elements and their Gibbs free energies foroxide formation will be provided below.

FIG. 1A shows zinc 106 enriched at interface 108 since zinc 106 has apositive Gibbs free energy for oxide formation compared to that ofaluminum. This enrichment of zinc 106 is an event that occurs during theanodizing of substrate 102. The presence of zinc 106 has been found toweaken the adhesion between anodic oxide 104 and substrate 102. That is,the presence of zinc 106 at interface 108 can cause anodic oxide 104 tobe susceptible to spalling, chipping, or peeling away from substrate102. It should be noted that other elements such as nickel and tin (notshown) have also been shown to have similar interfacial weakeningeffects when positioned at interface 108. In contrast, elements such ascopper and gold (not shown) at interface 108 can strengthen the adhesionof anodic oxide 104 to substrate 102.

The enrichment of zinc 106 at interface 108 during the anodizing processcannot generally be avoided by changing parameters of the anodizingprocess, or by using a typical pre-treatment operation. The enrichmentof zinc 106 is a consequence, primarily of the higher Gibbs free energyof oxide formation for zinc 106 compared to the aluminum metal ofaluminum matrix 112, and because zinc 106 is less readily oxidized thanthe aluminum metal of aluminum matrix 112. That is, the aluminum willoxidize in preference over zinc 106 during the anodizing process,resulting in interfacial enrichment of zinc 106 until equilibrium isachieved. It should be noted, that magnesium (not shown) that is acommon alloying element used in combination with zinc 106 in highstrength 7000-series alloys, has a low Gibbs free energy of oxidationand is preferentially oxidized over aluminum, resulting in noaccumulation of magnesium at interface 108.

FIG. 1B shows additional events that occur during the anodizing process.As described above, anodizing processes such as Type II and Type IIIanodizing processes (as defined by Military Specification Anodizing(MIL-A-8625)) involve the use of a sulfuric acid solution as anelectrolyte. It has been shown that sulfur species 114 originating fromthe sulfuric acid solution become significantly incorporated withinanodic oxide 104. Sulfur species 114 can be any sulfur-containingspecies formed during an anodizing process using a sulfur-containinganodizing solution, such as sulfate and/or sulfide ions. Duringanodizing, sulfur species 114 move toward substrate 102 and interface108, as indicated by arrows 115, driven by the applied electric currentfor anodizing. When sulfur species 114 reaches interface 108, they cancombine with enriched zinc 106 to form zinc-sulfur species 116, whichcan be any zinc and sulfur-containing species such as zinc sulfateand/or zinc sulfide.

Zinc-sulfur species 116 have been found to further weaken interfacialadhesion between anodic oxide 104 and substrate 102, making anodic oxide104 even more susceptible to spalling, chipping, or peeling. This can beparticularly problematic in zinc-rich aluminum alloys, such as some7000-series alloys, due to the relatively high levels of zinc 106. Inthis way, both zinc 106 and zinc-sulfur species 116 can be referred toas interface-weakening species. It should be noted that other elementscould combine with sulfur species 114 to form sulfates and/or sulfitesthat can weaken adhesion of anodic oxide 104 to substrate 102. Forexample, nickel can also form sulfates that detract from adhesion ofanodic oxide 104, whereas copper, silver, gold, and various otherelements can form oxides in preference over sulfates and generally donot form egregious interfacial sulfates. Thus, the term“interface-weakening species” is not limited to zinc 106 and zinc-sulfurspecies 116, but can refer generally to species that weakens theadhesion of anodic oxide 104 to substrate 102.

FIG. 1C shows graph 120 and image 122 indicating evidence of interfacialzinc enrichment for an anodized zinc-containing aluminum alloysubstrate. Graph 120 represents data collected by electron energy lossspectroscopy (EELS) and image 122 is collected using high-resolutionmicroscopy. In graph 120 and image 122, upper portions correspond to ananodic oxide (Ano) and lower portions correspond to an aluminum alloysubstrate (Al), with the dashed line labeled “ano-Al interface”corresponding to an interface region between anodic oxide (Ano) andaluminum alloy substrate (Al). The EELS graph 120 corresponds to a 20nanometer scan (indicated in image 122) taken across the ano-Alinterface. Graph 120 shows lines for oxygen (O) and zinc (Zn),corresponding to relative amounts of oxygen (O) and zinc (Zn) across theano-Al interface. As shown, oxygen (O) dramatically increases at theano-Al interface since this corresponds to the transition from metalmaterial (Al) to metal oxide (Ano). Graph 120 also clearly shows thatzinc (Zn) accumulates at the ano-Al interface.

As described above, the presence of copper within an aluminum alloy canstrengthen the adhesion of an anodic oxide. In fact, many commerciallyavailable aluminum-zinc alloys include significant amounts of copper asan alloying element. However, anodizing such commercially availablecopper-containing alloys results in entraining the copper into andseverely discoloring the resultant anodic oxide such that the anodicoxide takes on a distinctly yellow hue. Since the anodic oxide ispartially transparent, this can impart a yellow hue to thesilvery-colored base metal substrate (e.g., as viewed from surface 101of part 100 in FIGS. 1A and 1B), which can detract from the cosmeticappeal of the part.

This yellowing can be measured using conventional techniques such ascolorimetry using a spectrophotometer, and described according to acolor space such as CIE 1976 L*a*b* with a corresponding standardilluminant and white spot such as CIE Standard Illuminant D65. Theanodized substrate can be measured while in a non-dyed state—that is,without any color additives such as anodic dyes (e.g., organic ormetallic dyes). Note that the D65 (daylight) white spot is used as thereference throughout this document, but F2 (cool white fluorescent) andA (tungsten) will yield similar results, with the colors all fallingwithin approximately 0.1 b* in the region of interest, regardless ofwhich illuminant standard is used.

In general, L*a*b* color space is a model used to characterize colors ofan object according to color opponents L* corresponding to an amount oflightness, a* corresponding to amounts of green and magenta, and b*corresponding to amounts of blue and yellow. By convention, higher L*values correspond to greater amounts of lightness and lower L* valuescorrespond to lesser amounts of lightness. Negative b* values indicate ablue color, with more negative b* values indicating a bluer color, andpositive b* values indicate a yellow color, with more positive b* valuesindicating a yellower color. Anodic oxide 104 having b* values greaterthan 1 will generally have a perceptibly yellow hue. The presence of toomuch copper or other certain types of alloying elements within substrate102 and cause part 100 to have b* values greater than 1 when anodicoxide 104 is more than five micrometers in thickness.

Methods described herein can be used to strengthen the bond betweenanodic oxide and underlying aluminum alloy substrate withoutsubstantially yellowing the anodic oxide. A first strategy involvesusing a class of elements that become enriched at the interface morefavorably than zinc and/or other alloying elements associated withdelamination. Preventing or reducing the enrichment of these elements atthe interface during the anodizing process eliminates or reduces theformation of interface-weakening species at the interface. This firststrategy is referred to below as an interface-weakening speciesenrichment prevention strategy.

A second strategy involves using a class of elements, referred to assulfur-scavenging elements, which can bind with the sulfur speciesoriginating from an anodizing solution during anodizing. This preventsor reduces the occurrence of zinc and/or other elements associated withdelamination from combining with the sulfur species to form theinterface-weakening species (e.g., zinc-sulfur species 116) at theinterface. This second strategy is referred to below as asulfur-scavenging strategy. In some embodiments, a combination ofenrichment prevention and sulfur-scavenging strategies are used. Theseand other embodiments are described below.

Interface-Weakening Species Enrichment Prevention

One way of increasing the bond strength between an anodic oxide andhigh-strength aluminum alloy substrate is by preventing or reducing theenrichment of zinc at the interface between the anodic oxide andsubstrate that would otherwise occur during anodizing. Since zinc canact as an interface-weakening agent, preventing or reducing accumulationof zinc at the interface can increase the adhesion of the anodic oxideto the substrate. Furthermore, if interfacial zinc accumulation isavoided or reduced, this also prevents or reduces the formation ofzinc-sulfur compounds at the interface, which is also aninterface-weakening agent.

Preventing or reducing zinc enrichment can be accomplished by adding oneor more additional elements to the substrate that will enrich at theinterface in preference to zinc. To illustrate, FIG. 2 shows across-section view of part 200 formed using such an enrichmentprevention strategy. Part 200 includes aluminum alloy substrate 202 withanodic oxide 204 formed from a sulfur-containing bath (e.g., sulfuricacid-based bath) anodizing process. Anodic oxide 204 includes pores 210formed during the anodizing process. Substrate 202 includes alloyingelement zinc 206, which is incorporated within aluminum matrix 212.Substrate 202 can also include magnesium (not shown for simplicity) thatcan combine with zinc 206 form MgZn₂ precipitates within substrate 202to give substrate 202 high tensile strength, as described above. Sulfurspecies 214 originating from the sulfur-containing anodizing solutionbecomes incorporated within anodic oxide 204 during the anodizingprocess.

In addition to zinc 206 (and optionally magnesium), aluminum alloysubstrate 202 includes micro-alloying element 216, which is an elementthat enriches at interface 208 more favorably than zinc 206, andconsequently reduces or eliminates enrichment of zinc 206 at interface208. Micro-alloying element 216 is added in very small concentrations,i.e., less than 0.1 weight %, and in some embodiments preferably in aconcentration of 0.02 weight % to 0.05 weight %. These lowconcentrations have been found to be sufficient to inhibit zinc 206enrichment at interface 208, without significantly yellowing anodicoxide 204 or negatively altering other alloy properties of substrate 202such as strength, elongation, electrical or thermal conductivity, and/orcorrosion resistance.

The types of micro-alloying element 216 that preferably enrich atinterface 208 compared to zinc 206 can be identified based on how easilymicro-alloying element 216 oxidizes compared to zinc 206. That is, sincethe interfacial enrichment of micro-alloying element 216 duringanodizing is primarily a consequence of higher Gibbs free energies ofoxide formation compared to that of aluminum (of aluminum matrix 212),it may be assumed, to first approximation, that elements with higherGibbs free energies for oxide formation than zinc 206 will, in turn bepreferentially enriched at interface 208 over zinc 206. Thisapproximately limits elements of interest as possible candidates formicro-alloying element 216 to vanadium, phosphorus, tin, tungsten, iron,germanium, cadmium, molybdenum, nickel, cobalt, phosphorus, antimony,bismuth, arsenic, indium, tellurium, copper, thallium, osmium, selenium,iridium, mercury, platinum, silver, and gold (ranked in approximateorder of increasing Gibbs free energy for oxide formation, andconsequent enrichment relative to zinc 206).

FIG. 3 shows graph 300 indicating interfacial enrichment of a number ofelements as a function of Gibbs free energy (ΔG⁰). Graph 300 is amodified version of data provided in Corrosion Science, Vol. 39, No. 4,pp. 731-737 (1997). The x-axis of graph 300 indicates Gibbs free energy(ΔG⁰) for oxide formation of each element. The y-axis of graph 300indicates an amount of enrichment of each element at the interfacebetween an anodic film and aluminum alloy substrate, expressed in atoms(×10¹⁵) per cm². Graph 300 indicates that vanadium (V), tin (Sn), nickel(Ni), molybdenum (Mo), bismuth (Bi), antimony (Sb), indium (In), copper(Cu), mercury (Hg), silver (Hg), and gold (Au) have higher ΔG⁰ for oxideformation than zinc (Zn), and that these elements also enrich at theinterface. This indicates that using higher ΔG⁰ for oxide formationcompared to that of zinc is a good first approximation for determiningtypes of micro-alloying elements that can accumulate at the anodicoxide-substrate interface.

FIG. 4 shows graph 400 and image 402 indicating evidence of preventionof interfacial zinc enrichment when using copper (Cu) as amicro-alloying element. Graph 400 represents data collected by electronenergy loss spectroscopy (EELS) and image 402 is collected usinghigh-resolution microscopy. In graph 400 and image 402, upper portionscorrespond to an anodic oxide (Ano) and lower portions correspond to analuminum alloy substrate (Al), with the dashed line labeled “ano-Alinterface” corresponding to an interface region between anodic oxide(Ano) and aluminum alloy substrate (Al). The EELS graph 400 correspondsto a 20 nanometer scan (indicated in image 402) taken across the ano-Alinterface. Graph 400 shows lines for oxygen (O), zinc (Zn), and copper(Cu) corresponding to relative amounts of oxygen (O), zinc (Zn), andcopper (Cu) across the ano-Al interface. As shown, copper (Cu)accumulates at the ano-Al interface while zinc (Zn) does not. This EELSscan confirms that copper (Cu) micro-alloying element preferentiallyenriches at the ano-Al interface over zinc (Zn), and can result no zinc(Zn) enrichment at the ano-Al interface.

Some elements may be eliminated as candidates for a micro-alloyingelement for various reasons. For example, phosphorus is known to weakenthe interface between an anodic oxide and substrate, and can thereforebe avoided. Lead, mercury, cadmium, thallium, and arsenic may be avoideddue to their toxicity, whilst, nickel may be undesirable forapplications where skin contact is anticipated. Mercury, bismuth, lead,tin, cadmium, and indium may not constitute practical alloying elementsfor aluminum due to their low melting points or phase changes occurringwithin a typical aluminum alloy's thermal processing window.

Assuming that the micro-alloying element is uniformly distributed withinaluminum alloy substrate (and preferably within the aluminum matrix)rather than in discrete second phase particles (which can themselves bea cause of cosmetic defect in anodizing), solubility in the aluminummatrix may also be a selection criterion. Iron, for example, can formAl₁₃Fe₄ precipitates, which act as a grain refiner, limiting graingrowth during thermo-mechanical processing of the alloy. This may inturn be perceived as a cosmetic defect in the anodic oxide. Assumingthat a solubility of about 0.05 weight % or more in the aluminum matrixis a further condition can eliminate platinum, palladium, selenium,tellurium, arsenic, antimony, nickel, cobalt, molybdenum, andtungsten—although some of these elements will be considered and exploredin some cases.

Elements such as tungsten, germanium, tellurium, osmium, selenium,iridium, rhodium, platinum, palladium, silver, and gold may be lessdesirable candidates due to their scarcity or cost. However, since theymay only be needed in low concentrations, they may be considered in somecases. Of the remaining elements having higher Gibbs free energies foroxide formation than zinc identified above, including the rare orexpensive metals, some, such as copper and gold, enhance precipitationstrengthening, and can therefore possibly be used in combination withlower amounts of zinc.

A major consideration for applications that are to be used for cosmeticsurface finishes of consumer products is the intrinsic color of thesurface of the part, including the anodic oxide, after anodizing.Elements such as iron, copper, and silver can discolor the anodic oxide.As described above, copper, in particular, results in adding a yellow orbronze color to the anodic oxide. This yellow discoloration isnoticeable even when copper is added in quantities as low as about 0.1weight % to about 0.2 weight %, with b* values of greater than 3 whenthe anodic oxide has a thickness of 10 micrometers or more usingprocessing conditions of a typical Type II anodizing process. Typicalzinc-magnesium-copper aluminum alloys such as commercially availablealuminum alloy 7010 (with 1.5-2.0 weight % copper) and 7075 (with1.2-2.0 weight % copper) have severely discolored anodic oxide (b*>>1).This makes anodized 7010 and 7075 aluminum alloys unsuitable for use incertain products, where a silvery-colored aluminum appearance isdesired.

Other commercially available zinc and magnesium alloying element-based7000-series alloys specify maximum levels of copper: notably 0.2 weight% and 0.1 weight % for 7003 and 7005 respectively. But such permittedlevels would still be too high for a desired degree of color control(i.e., b*<1), especially as other elements such as manganese aresimilarly tolerated or specified as 0.3 weight % max and 0.2-0.7 weight% in 7003 and 7005, respectively. The anodic oxide film thickness onsuch alloys could be restricted to just a couple of micrometers tominimize discoloration, but that approach severely limits the processwindow for anodizing parts, and consequently limits the wear andcorrosion protection offered by the anodic oxide.

To achieve a desirable level of high clarity (e.g., L*>80, andpreferably L*>85) and substantially colorlessness (e.g., b*<1, andpreferably b*<0.5) of an anodic oxide, formed under typical Type IIanodizing conditions to thicknesses of 10 micrometers or more, thealuminum alloy composition specification for a high-strength 7000-seriesalloy must, for example, specify that with the exception of zinc andcertain corresponding precipitate-forming strengthening element orelements (e.g., magnesium or lithium). For example, aluminum alloy canhave strict limits on all elements that would result in discoloration ofthe anodic oxide or cause other cosmetic defects. For example, limitsmight be set at 0.01 weight % maximum for chromium, copper, manganeseand zirconium, 0.02 weight % maximum for titanium, 0.05 weight % maximumfor silicon, 0.08 weight % maximum for iron and 0.01 weight % maximumfor any other non-specified element, to a total maximum concentration of0.1 weight % of other non-specified elements. Note that this range ofelemental composition is provided by way of example for yieldingsubstantially colorless anodic oxides, and are not intended to limit thepossibility of other variations that would fall within the scope ofinventive embodiments presented herein. That is, the concentrations ofchromium, copper, manganese, zirconium, titanium, silicon, iron, and/orother non-specified elements can be slightly varied from those listedabove and still achieve anodic oxides with acceptable levels of clarity.

Aluminum alloy substrate compositions without copper can offer maximumclarity and colorlessness of the anodic oxide. However, the absence ofcopper in these alloys can result in more egregious accumulation of zincat the interface, and necessitates the development of the alternativestrategies for delamination mitigation. In the present work, themicro-alloying element is added in controlled “micro-alloying” amounts(<0.1 weight %) to substrates made of high strength aluminum alloys,such as 7000-series alloys, for the specific purpose of eliminatinginterfacial enrichment of zinc and/or other delamination species. Themicro-alloying element is added in specified levels just sufficient toinhibit enrichment of zinc and/or other delamination species at theinterface without significant discoloration of the anodic oxide and theresulting surface finish of the part (i.e., unlike the commerciallyavailable 7003, 7005, and 7010 alloys), and without significantlyaltering other alloy properties of the substrate such as strength,elongation, electrical or thermal conductivity, or corrosion resistance.

The amount of micro-alloying element can depend on the type ofmicro-alloying element and on cosmetic requirements. For example, asdescribed above, even relatively small amounts of copper within aluminumalloy substrates have been found to result in discolored anodized part.To illustrate, FIG. 5A shows graph 500 indicating yellowing effects ofdifferent amounts of copper as a micro-alloying element to an aluminumalloy substrate. Graph 500 shows b* color space values forzinc-magnesium aluminum alloy substrates having different amounts ofcopper anodized using different anodizing bath temperatures. Data forthree type of zinc-magnesium aluminum alloy substrates having differentconcentrations of copper: 1A=0.05 weight % copper; 1B=0.1 weight %copper; and 1C=0.15 weight % copper. In addition to the copper, eachtype of aluminum alloy substrate 1A, 1B, and 1C include only magnesiumand zinc as alloying elements.

Each type of substrate 1A, 1B, and 1C was anodized using 1.5 A/dm²current density in 200 g/L sulfuric acid solution, to a target thicknessof 15 micrometers. Three different anodizing bath temperatures wereexplored: 17 degrees Celsius, 20 degrees Celsius, 23 degrees Celsius,and 26 degrees Celsius. Graph 500 indicates a linear relationshipbetween levels of copper and b* value, with higher concentrations ofcopper correlating with higher b* values. As described above, higher b*values correspond to yellower appearance—thus the more copper added, theyellower the appearance of the anodized part. Graph 500 also indicatesthat increasing anodizing bath temperatures can reduce values b*.However, higher anodizing temperatures can result in a softer the anodicoxide. Thus, higher anodizing temperatures may not be suitable forcertain applications. For good wear protection, a temperature of 20degrees Celsius is generally preferred, together with a thickness of 10micrometers or more. Graph 500 indicates that in embodiments wherezinc-magnesium aluminum alloy substrates are required to have b* valuesno more than 1, the copper concentration should not exceed about 0.1weight %, depending, in part, on the temperature of the anodizing bath.Similarly, lower current density may be used to reduce the yellowing,but the effect is not as strong as that of raising temperature, andreduced current density again softens the resulting anodic oxide, with1.5 A/dm² being preferred for sufficient hardness at room temperature.

An additional consideration in determining how much micro-alloyingelement should be used is the commercial recyclability of the part. Byrestricting the level of the micro-alloying element to 0.05 weight % orless, there is no implication for recycling of the part. This is becausemost commercial 7000-series alloys specify a maximum of 0.05 weight %for “other” alloying elements and could therefore accommodate the parthaving micro-alloying element at levels of 0.05 weight % or less. Thus,in some embodiments, the micro-alloying element is preferably added inamounts less than about 0.05 weight %. This is particularly true of lesscommon alloying element candidates (such as silver, antimony), and isgenerally not a limiting factor for copper, which is used in most7000-series alloys at levels of at least 0.5 weight %.

In one preferred embodiment, a specified micro-alloying addition ofbetween 0.02 weight % and 0.05 weight % copper is made to an aluminumalloy that includes about 5.5 weight % zinc and about 1 weight %magnesium, with substantially no other alloying elements. Thiscomposition corresponds to a relatively pure and balancedaluminum-zinc-magnesium 7000-series alloy optimized for a high yieldstrength (about 340-350 MPa), hardness (about 125 HV), andheat-treatability, with a very small amount of copper. It has been foundthat anodizing this substrate composition using Type II anodizing (200g/L sulfuric acid bath, at 20 degrees Celsius, with 1 A/dm² currentdensity) to form an anodic oxide having a thickness of about 15micrometers completely eliminates delamination of the anodic oxide, asassessed by rock tumble testing or by indentation testing such thatdescribed in U.S. application Ser. No. 14/678,881, which is incorporatedby reference herein in its entirety. The color of the anodized surfaceof this substrate composition remains silvery, with a b* value of about0.4, corresponding to very little yellowing.

Similar results were obtained with silver as the micro-alloying element,although for a 0.05 weight % addition, the corresponding atomic % ofsilver is lower than copper, and the delamination resistance was not asimproved as found with copper. In some embodiments, approximately 0.1weight % silver micro-alloying element is required to eliminatedelamination in a 15 micrometer thick anodic oxide, and at that level,the discoloration is found to be higher than that using copper, albeitb* is still less than 1.

In some embodiments, the micro-alloying element includes a combinationof copper and silver so as to give compounded benefits in terms ofdelamination resistance, but also compounded color shifts. One or moreof germanium, osmium, iridium, rhodium, and gold may be used similarlyto silver and copper, in some cases even without any correspondingyellow discoloration. However the cost premium over silver may makethese elements less desirable.

Vanadium was also evaluated, even though its Gibbs free energy for oxideformation is very close to that of zinc. A very slight improvement indelamination resistance was observed, when using 0.05 weight % vanadiumconcentration, and there was no yellow significant discoloration.Titanium, and zirconium were also evaluated. However, in one embodiment,titanium and zirconium showed no ability to reduce interfacialenrichment of zinc and no significant improvement in delaminationresistance, even at 0.3 weight %. This is consistent with the Gibbs freeenergies for oxide formation for each of titanium and zirconium beinglower than that of zinc.

Of the elements as candidates for the micro-alloying element that mightbe dismissed on the basis of limited solubility in the aluminum matrix,nickel, molybdenum, and antimony were explored at 0.05 weight %. Nickelwas detrimental—perhaps forming even worse interfacial compounds thanzinc. Molybdenum was of no significant benefit—possibly because itsGibbs free energy for oxide formation is not far enough from that ofzinc, compounded by the fact that its relatively high atomic weightmakes its corresponding atomic concentration low. Antimony was ofsignificant benefit—comparable to silver, but without the undesirableyellow discoloration of silver. Instead, antimony gave a slight bluediscoloration, with a b* value of −0.2. This could possibly be used incombination with a yellowing element to neutralize color. However,antimony can introduce small spherical inclusions to the anodicoxide—probably corresponding to aluminum-antimony precipitates in thealloy, which inhibit growth of a completely uniform anodic oxide film.However, it is possible for more than one type of micro-alloying elementto be used to achieve a cumulative effect in offsetting interfacialenrichment of zinc and/or discoloration of the anodic oxide. Forexample, an element that results in adding a yellow hue (b*>0) to theanodic oxide, such as copper, can be added to increase interfacialenrichment of zinc 206, while an element that results in adding a bluehue (b*<0) to the anodic oxide, such as antimony, can be added to offsetthe yellowing of the copper. In some embodiments, a target b* for thepart is between −1 and 1. In some embodiments, a target b* for the partis between −0.5 and 0.5.

Given the above-described considerations and limitation, in someembodiments, preferred candidates for the micro-alloying element includeone or more of vanadium, germanium, cobalt, antimony, copper, tellurium,osmium, selenium, iridium, rhodium, palladium, silver, and gold. In someembodiments, preferred candidates for the micro-alloying element includeone or more of copper, silver, and antimony. In some embodiments, morethan one element is used in combination. In some embodiments, themicro-alloying element is added at levels of no more than about 0.1weight %. In some embodiments, the micro-alloying element is preferablyadded at levels of between 0.02 weight % and 0.05 weight %. In someembodiments, the anodized part, as viewed from an exposed surface of theanodic oxide (surface 201), has a b* value of less than 2, in someembodiments a b* value less than 1, even when anodized to thicknesses of10 micrometers or more, at current densities of 1 A/dm² or more and ananodizing bath temperature of 25 degrees Celsius or less. In someembodiments, the anodized part has an L* value (corresponding to a levelof brightness) greater than 75, and in some preferred embodiments, theanodized part has an L* value greater than 85.

In one preferred embodiment, the specified addition of copper as amicro-alloying element at a level of between 0.02 and 0.05 weight % isnotably within the typical tolerances for copper as an impurity incertain commercially available 7000-series alloys, such as 7003 and7005, which respectively specify 0.1 and 0.2 weight % maximum forcopper. A crucial distinction between such tolerated impurity levels incommercially available 7000-series alloys and the specifiedmicro-alloying additions made in embodiments presented herein, however,is the specified micro-alloying addition. In particular, in embodimentspresented herein, copper has both a specified minimum (e.g., 0.02 weight%) to ensure sufficient interfacial adhesion of the anodic oxide, and aspecified maximum (e.g., 0.05 weight %) that is carefully selected tolimit the maximum discoloration of the anodic oxide, and as such is alsogenerally lower than any commercial high strength 7000-series alloys'tolerated impurity level for copper.

There can be other advantages of using micro-alloying element within thealuminum alloy substrates. For example, one problem encountered in theanodizing of aluminum alloys only having zinc and magnesium as alloyingelements is the differential growth of an anodic oxide on grains ofdifferent surface orientation within the substrate, resulting in graintexture-related thickness variation. In the presence of zinc, grains of{111} surface orientation are relatively anodic, as compared to grainsof {110} and {100} orientation, and are thus anodized anomalously fast.This can detract from the aesthetics of the anodized finish of apart—notably as apparent pits at the anodic oxide and substrateinterface.

One solution to the problem of differential growth rates for grains ofdifferent surface orientation is described in U.S. application Ser. No.14/678,868, which is incorporated by reference herein in its entirety.In the U.S. application Ser. No. 14/678,868, an electrolyte to enableanodizing at low current density and/or increased temperature whilstmaintaining adequate anodic oxide hardness is described. In the presentapplication, the micro-alloying element can induce faster growth of theanodic oxide in other orientations (e.g., {110} and/or {100}) tomitigate the discrepancy. For example, copper micro-alloying element hasshown to induce faster anodic oxide grown on grains of {110}orientation. In one embodiment, 0.05 weight % copper proved sufficientto dilute the cosmetic defect observed when Type II anodizing (200 g/Lsulfuric acid with 1.5 A/dm² current density) beyond the limit ofperception—even on a substrate with a mirror-lapped surface anodized toa thickness of 10 micrometers or more. This corresponds to a thicknessdiscrepancy of less than 5% between grains of distinct orientation—farless than the typical 10% thickness discrepancy that would result in theabsence of the micro-alloying addition. In this way, the presence of amicro-alloying element can also reduce non-uniform growth of the anodicoxide and related cosmetic defects.

FIG. 5B shows graph 510 indicating the effect of adding 0.05 weight %copper on a thickness uniformity of a resultant anodic oxide. Graph 510shows that the absence of the copper addition (5.5 Zn, 1.0 Mg sample),the presence of zinc in the alloy results in very non-uniform growthrates for different crystallographic orientations, with surfaces oforientations close to {111} orientation in particular growing at anaccelerated rate. This results in grains of {111} orientation having anoxide film of about 3-9% thicker than the average filmthickness—appearing as distinct “pit”-like features in the anodic oxide.These are particularly evident on a substrate that has been lapped to amirror finish. The addition of 0.05 weight % copper (5.5 Zn, 1.0 Mg,0.05 Cu sample) is sufficient to overcome preferential growth of oxideon {111} surfaces and to ensure film thickness uniformity within 5%,without having to resort to such methods as that disclosed in U.S.patent application Ser. No. 14/678,868, which is incorporated herein inits entirety.

FIG. 6A shows flowchart 600, illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate. At602, a micro-alloying element is incorporated into an aluminum alloysubstrate. The aluminum alloy substrate can include zinc and magnesiumalloys that give the substrate high tensile strength. In someembodiments, the substrate is an enclosure, or part of enclosure, for anelectronic device. The micro-alloying element is added in an amount thatis less than conventionally used for alloying purposes. In someembodiments, a concentration of the micro-alloying element within thealuminum alloy substrate is at most 0.1 weight %, and in someembodiments about 0.05 weight % or less. The micro-alloying element canbe characterized as having a higher Gibbs free energy of oxide formationthan the zinc. In some embodiments, the micro-alloying element includesone or more of vanadium, tin, nickel, molybdenum, germanium, bismuth,cobalt, antimony, tellurium, osmium, selenium, indium, iridium, rhodium,palladium, copper, mercury, silver, and gold. In some embodiments, themicro-alloying element includes one or more of copper, silver, andantimony. In particular embodiments, two or more types of micro-alloyingelements are used, such as copper and silver.

At 604, the aluminum alloy substrate is anodized. In some embodiments,the anodizing takes place in an anodizing solution comprising sulfuricacid. In some embodiments, a type II anodizing process is used. Duringanodizing, the micro-alloying element enriches at an interface betweenthe substrate and a resultant anodic oxide, thereby preventing orreducing enrichment of zinc at the interface. Since zinc can be aninterface-weakening species, prevention or reduction of zincaccumulation at the interface can increase an adhesion strength of theanodic oxide to the substrate. Furthermore, this prevents or reducesformation of zinc-sulfur species, another interface-weakening species,at the interface.

FIG. 6B shows flowchart 610 illustrating a process of reducinggrain-related defects in an anodized high-strength substrate. Thealuminum alloy substrate can include zinc and magnesium alloys that givethe substrate high tensile strength. At 612, a micro-alloying elementhaving a higher Gibbs free energy of oxide formation than the zinc isincorporated into an aluminum alloy substrate. A concentration of themicro-alloying element within the aluminum alloy substrate can be atmost 0.1 weight %, and in some embodiments about 0.05 weight % or less.In some embodiments, the micro-alloying element includes one or more ofvanadium, tin, nickel, molybdenum, germanium, bismuth, cobalt, antimony,tellurium, osmium, selenium, indium, iridium, rhodium, palladium,copper, mercury, silver, and gold. In some embodiments, themicro-alloying element includes one or more of copper, silver, andantimony. In particular embodiments, two or more types of micro-alloyingelements are used, such as copper and silver.

At 614, the aluminum alloy substrate is anodized, using for example, asulfuric acid anodizing solution. In some embodiments, a type IIanodizing process is used. During anodizing—even at 1.5 A/dm² in asulfuric acid solution, the presence of the micro-alloying elementreduces the discrepancy between the growth rates of anodic oxide ongrains of {111} orientation and other orientations. This may be becausethe micro-alloying element increases the growth rate of the anodic oxideon surface orientations other than {111}—such as {110} and {100} grainorientations. It may also depress the anomalously high growth rate of{111} oriented grains. The result is an oxide with thickness uniformityto within 2-3% between grains of distinct orientations—far less than the10% discrepancy that would result in the absence of the micro-alloyingaddition. The resultant anodized substrate is free from pitting defectsrelated to accelerated anodic oxide grown at {111} grains. This can beespecially important in anodized substrates that have an underlyingpolished and highly uniform surface (e.g., having a mirror shine).

Sulfur-Scavenging

Another way of increasing the bond strength between an anodic oxide andhigh-strength aluminum alloy substrate is by preventing or reducing theoccurrence of sulfur bonding with zinc during anodizing, therebypreventing or reducing formation of zinc-sulfur species at theinterface. As described above, zinc-sulfur species can act as aninterface-weakening species—thus, eliminating or minimizing theformation of such zinc-sulfur can increase the adhesion strength of theanodic oxide.

This can be accomplished by adding different class of alloying elementsto aluminum alloy substrate that will preferentially bond with sulfur,thereby preventing the sulfur from bonding with zinc. To illustrate,FIG. 7 shows a cross-section view of part 700 formed using such asulfur-scavenging strategy. Part 700 includes aluminum alloy substrate702 with anodic oxide 704 formed from a sulfur-containing bath (e.g.,sulfuric acid-based bath) anodizing process. Pores 710 of anodic oxide704 are formed during the anodizing process. Substrate 702 includesalloying element zinc 706 and optionally magnesium (not shown forsimplicity) that can combine with zinc 706 form precipitates such asMgZn₂ and give substrate 702 high tensile strength, as described above.Sulfur species 714 originating from the sulfur-containing anodizingsolution becomes incorporated within anodic oxide 704 during theanodizing process. Sulfur species 714 is likely in ionic form, such as asulfide, and possibly compounded with oxygen ions as a sulfate and/orsulfide ion.

Substrate 702 includes sulfur-scavenging species 705 that has a strongaffinity for bonding with sulfur species 714, and therefore readilybonds with sulfur species 714 forming bound sulfur species 707. In thisway, sulfur-scavenging species 705 “scavenges” inward-diffusing sulfurspecies 714 and prevents sulfur species 714 from reaching the zinc 706at interface 708. Bound sulfur species 707 becomes locked within anodicoxide 704 and away from interface 708, and thus does not interfere withthe adhesion capability of anodic oxide 704 to substrate 702.

Criteria for choosing sulfur-scavenging species 705 include how readilyoxidized it is, its affinity for sulfur species 714, and its ionicmobility. Sulfur-scavenging species 705 should be more readily oxidizedthan aluminum such that, sulfur-scavenging species 705 is oxidizedtogether with aluminum matrix 712 during anodizing. Table 1, below,lists a number of elements based on their calculated Gibbs free energyfor oxide formation (−ΔG⁰ (kCal/mol O₂)).

TABLE 1 Oxide Formation Energies Element Oxide −ΔG⁰ (kCal/mol O₂)Yttrium (Y) Y₂O₃ 320 Calcium (Ca) CaO 320 Scandium (Sc) Sc₂O₃ 310Europium (Eu) EuO 290 Gadolinium (Gd) Gd₂O₃ 287 Magnesium (Mg) MgO 286Lanthanum (La) La₂O₃ 285 Lithium (Li) LiO₂ 285 Cerium (Ce) Ce₂O₃ 285Strontium (Sr) SrO 280 Aluminum (Al) Al₂O₃ 277 Hafnium (Hf) HfO₂ 265Zirconium (Zr) ZrO₂ 262 Erbium (Er) Er₂O₃ 260 Titanium (Ti) Ti₂O₃ 242Silicon (Si) SiO₂ 217 Tantalum (Ta) Ti₂O₅ 198 Vanadium (V) VO 198Manganese (Mn) MnO 184 Chromium (Cr) Cr₂O₃ 179 Niobium (Nb) NbO₂, NbO176, 187 Zinc (Zn) ZnO 166 Rubidium (Rb) Rb₂O 158 Indium (In) In₂O₃ 157Tin (Sn) SnO₂ 139 Tungsten (W) WO₃ 133 Iron (Fe) Fe₃O₄ 130 Germanium(Ge) GeO₂ 129 Molybdenum (Mo) MoO₂, MoO₃ 127, 106 Cobalt (Co) CoO 115Nickel (Ni) NiO 114 Antimony (Sb) Sb₂O₃ 111 Bismuth (Bi) Bi₃O₄ 104Copper (Cu) Cu₂O 80 Tellurium (Te) TeO₂ 77 Thallium (Tl) Tl₂O 69 Osmium(Os) OsO₂ 62 Selenium (Se) SeO₂ 54 Iridium (Ir) IrO₃ 43 Rhodium (Rh)Rh₂O₃ 42 Platinum (Pt) Pt₃O₄ 22 Silver (Ag) Ag₂O 14

Table 1 indicated that yttrium (Y), calcium (Ca), scandium (Sc),europium (Eu), gadolinium (Gd), magnesium (Mg), lanthanum (La), lithium(Li), cerium (Ce), and strontium (Sr) have more negative ΔG⁰ for oxideformation than aluminum (Al), and thus are more readily oxidized thanaluminum (Al).

Another consideration for choosing viable candidates for thesulfur-scavenging species is an element's ability to form a stablecompound with sulfur species 714. As described above, sulfur-scavengingspecies 705 can bind with sulfur species 714 to form a sulfate, asulfide or other suitable stable bound sulfur species 707. Thus, oneapproximation as to an element's ability to bond with sulfur species 714can be its enthalpy of sulfate formation.

Table 2, below, lists a number of elements based on enthalpy (−ΔH) ofsulfate formation.

TABLE 2 Sulfate Formation Energies Element Sulfate −ΔH (kJ/mol SO₄)Barium (Ba) BaSO₄ 1473 Radium (Ra) RaSO₄ 1471 Strontium (Sr) SrSO₄ 1453Lithium (Li) LiSO₄ 1437 Rubidium (Rb) RBSO₄ 1436 Calcium (Ca) CaSO₄ 1435Sodium (Na) Na₂SO₄ 1387 Cerium (Ce) Ce₂(SO₄)₃ 1318 Magnesium (Mg) MgSO₄1285 Beryllium (Be) BeSO₄ 1250 Aluminum (Al) Al₂(SO₄)₃ 1147 Hafnium (Hf)Hf(SO4)₂ 1115 Zirconium (Zr) Zr(SO₄)₂ 1109 Manganese (Mn) MnSO₄ 1065Zinc (Zn) ZnSO₄ 982 Chromium (Cr) Cr₂(SO₄)₃ 970 Iron (Fe) FeSO₄ 929Palladium (Pd) PdSO₄ 920 Cadmium (Cd) CdSO₄ 935 Cobalt (Co) CoSO₄ 888Iron (Fe) Fe₂(SO₄)₃ 861 Bismuth (Bi) Bi₂(SO₄)₃ 848 Nickel (Ni) NiSO₄ 872Copper (Cu) Cu₂O 771 Gold (Ag) Ag₂SO₄ 716

Table 2 indicates that barium (Ba), radium (Ra), strontium (St), lithium(Li), rubidium (Rb), calcium (Ca), sodium (Na), cerium (Ce), magnesium(Mg), beryllium (Be), aluminum (Al), hafnium (Hf), zirconium (Zr), andmanganese (Mn) each have very large negative (i.e., exothermic) valuesfor ΔH for sulfate formation, and are therefore strong sulfate formers.Barium (Ba), radium (Ra), strontium (St), lithium (Li), rubidium (Rb),calcium (Ca), sodium (Na), cerium (Ce), magnesium (Mg), beryllium (Be)have more negative ΔH for sulfate formation than aluminum (Al), andtherefore may be preferable in some embodiments.

In some preferred embodiments, the criterion for choosing thesulfur-scavenging species is based on both oxide formation and sulfateformation. Based on Tables 1 and 2, these elements include one or moreof lithium (Li), magnesium (Mg), strontium (Sr), and calcium (Ca). Itshould be noted, however, that this does not necessarily include allpossible suitable sulfur-scavenging species.

FIG. 8 shows an annotated periodic table illustrating some criteria forchoosing a suitable sulfur-scavenging species, in accordance with someembodiments. Elements that have Gibbs free energy (ΔG⁰) for oxideformation greater than or equal to ΔG⁰ for aluminum oxide (Al₂O₃)formation can be eliminated. This leaves lithium (Li), beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra),scandium (Sc), yttrium (Y), lutetium (Lu), lawrencium (Lr), lanthanum(La), cerium (Ce), samarium (Sm), europium (Eu), gadolinium (Gd),actinium (Ac), and thorium (Th).

Some elements indicated in FIG. 8 can be further eliminated due to costor other factors, such as toxicity or radioactivity (as indicated). Forexample, the cost of beryllium (Be), radium (Rd), scandium (Sc), yttrium(Y), lutetium (Lu), lawrencium (Lr), actinium (Ac), thorium (Th),samarium (Sm), europium (Eu), and gadolinium (Gd) may be too high formany practical purposes. Beryllium (Be), radium (Rd), yttrium (Y),lanthanum (La), actinium (Ac), cerium (Ce), and thorium (Th) may betoxic and/or radioactive. This leaves one or more of lithium (Li),magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) aspreferred sulfur-scavenging candidates, according to some embodiments.

An additional consideration for choosing a sulfur-scavenging species isits ionic mobility within the anodic oxide during anodizing. Thosespecies that are lighter in weight, such as lithium, sodium andmagnesium, have high mobility within an anodic oxide, thereby increasingthe likelihood of reaching and binding with the counter-flowing sulfurspecies. Moreover, their high ionic mobility relative to aluminumensures that under the applied electric field, they diffuse away fromthe metal oxide interface more rapidly than aluminum. Upon encounteringcounter-flowing sulfur species, they will form compounds away from theanodic oxide-substrate interface. They effectively present acounter-flowing barrier to the flow of such anions towards theinterface.

It should be noted, however, that ion mobility might not be the onlyfactor to consider. For example, Table 2 above shows that barium (Ba)forms a very strong bond with sulfate. However, the barium atom is muchheavier than elements such as sodium and magnesium, generally makingbarium less effective for a given weight percent of lighter elementssuch as lithium, sodium and beryllium. Never the less, barium's highersulfate energy formation may counterbalance its higher atomic weight.

Other considerations can include the solubility of the element withinaluminum and the abundance of the element. Ideally, the elements inquestion should also present solubility within the aluminum matrix tothe desired level, possibly also eliminating barium, calcium, cerium, orgadolinium in certain embodiments, For example, scandium is more solublein face-centered cubic aluminum (0.04 atomic % at 500 Celsius) comparedto yttrium (0.008 atomic % at 500 Celsius). However, this solubilitycondition may not be essential as being a suitable candidate, providedthat the element does not have an adverse effect on the aluminum alloy'smicrostructure or on the anodic oxide cosmetics.

Low stability or low melting point may also be considered, which mayrule out sodium, potassium, and rubidium. Elements such as scandium,europium, and hafnium are extreme scarce and may be further ruled outfor this reason. In some embodiments where the above factors areconsidered, preferable sulfur-scavenging species include one or more oflithium, magnesium, calcium, strontium, and barium. In some embodiments,preferable sulfur-scavenging species include one or more of lithium,magnesium, calcium, strontium, barium, scandium, and yttrium. In someembodiments, a combination of two or more of lithium, magnesium,calcium, strontium, barium, scandium, and yttrium are used.

Other considerations include the cosmetic effects of adding thesulfur-scavenging species. As described above, some elements cannoticeably discolor a resultant anodic oxide, with yellow discoloringparticularly undesirable in certain applications. Lithium, magnesium,calcium, strontium, and barium have no significant yellowing effect on aresultant anodic oxide in amounts sufficient to increase delaminationresistance, and can therefore be used without negatively affecting thecosmetic qualities of the anodic oxide.

The amount of sulfur-scavenging species can vary depending on the typeof sulfur-scavenging species. For example, it may be desirable to addthe sulfur-scavenging species to an amount sufficient to result in ananodic oxide having a predetermined determined delamination resistance,such as determined by techniques described in U.S. application Ser. No.14/678,881, which is incorporated herein in its entirety. It should benoted, however, that the amount of sulfur-scavenging species added tothe aluminum alloy to provide sufficient sulfur-scavenging capability issignificantly greater than the micro-alloying amounts described abovewith respect to preventing enrichment of interface-weakening species.For example, it has been found that 2 weight % or more of lithium shouldbe used in order to provide sufficient sulfur-scavenging and preventdelamination. However, high concentrations of alloying elements reducethe thermal conductivity of the alloy, which may be undesirable in someapplications. In some embodiments, the sulfur-scavenging species isadded to the substrate at a concentration ranging from about 0.5 weight% and about 3 weight %.

One alloying element that has proven to be a good sulfur-scavengingspecies is lithium. As described above, lithium added in a concentrationof about 2 weight % proved to eliminate delamination. Whilst interfacialenrichment of zinc is shown to occur, the counter-migration of lithiumthrough the anodic oxide is sufficient to limit the diffusion of sulfurspecies towards the interface, and to eliminate the formation of theinterface-weakening species of zinc and sulfur—even when the anodizingis conducted under conditions which would result in a very weakinterface in the absence of lithium (such as a conventional Type IIanodizing process conducted in 200 g/L sulfuric acid at 1.5 A/dm²current density and 20 degrees Celsius). Furthermore, if the addition oflithium is used in combination with other approaches to minimizedsulfate incorporation from the electrolyte, such as the mixed acidelectrolyte compositions described in U.S. application Ser. No.14/678,868 (which is incorporated herein in its entirety), a more robustsolution can be achieved, and one or more of the necessary conditionsmay be relaxed (e.g., use less lithium, or higher current density, orlower anodizing bath temperature).

Magnesium, in particular, as a sulfur-scavenging species can be ofinterest since most commercially available 7000-series aluminum alloysalready include magnesium and zinc as alloying elements. As describedabove, magnesium can be a key to the strengthening mechanism of many7000-series alloys by virtue of its propensity to form precipitates suchas MgZn₂, and specifically an η′ phase, within the aluminum matrix,within the aluminum matrix. FIG. 9 shows graph 900 indicating magnesiumand zinc concentrations of different commercially available 7000 seriesaluminum alloys: 7005, 7108, 7003, 7029, 7075, 7050, 7030, 7046A, 7046,as well as custom aluminum alloy composition 904, which is based onoptimal η′ precipitation strengthening, and custom aluminum alloycomposition 906. The x-axis of graph 900 indicates weight % of zinccontent and the y-axis indicates weight % of magnesium content withinthe aluminum alloys. Most of the commercial alloys include significantconcentrations of other alloying elements which are not shown: 7029,7030, 7046, 7050, and 7075, for example, all include copper at levelsthat would significantly discolor an anodic oxide.

Line 902 represents balanced zinc and magnesium compositions forproviding MgZn₂ precipitates for enhancing the strength of the aluminumsubstrate. That is, line 902 represents stoichiometric amounts of zincand magnesium to form MgZn₂η′ precipitates. Alloy compositions belowline 902 can be characterized as being zinc-rich, and alloy compositionsabove line 902 can be characterized as being magnesium-rich. Excess zincor magnesium in zinc-rich or magnesium-rich alloy compositions willreside in the aluminum matrix of the aluminum alloy, reducing thethermal or electrical conductivity of the alloy. Thus, for someapplications, and notably for electronics enclosures, which play a rolein dissipating heat, it is preferable to avoid this by choosing aluminumalloys having a generally balanced magnesium and zinc composition, suchas custom aluminum alloy composition 904. In addition to the Mg:Zn ratiodefined by the precipitate stoichiometry, an optimal level (volumefraction) of precipitate strengthening has informed the selection ofexact composition target 904, allowing for a given homogenization,quenching, extrusion and artificial ageing process to achieve a targetstrength, such as 340 MPa.

Since magnesium can also act as an effective sulfur-scavenging species,an excess of magnesium over the balanced composition of custom alloy 904can be made, such as indicated magnesium-rich custom alloy 906. That is,a magnesium-rich custom alloy 906 may be beneficial in providing theadded benefit of sulfur-scavenging and thereby improving adhesion of ananodic oxide. The amount of excess magnesium relative to zinc to providesuch sulfur-scavenging benefit can be significant. Namely the atomicconcentration of magnesium should be at least equal to half the atomicconcentration of zinc, and preferably equal to or greater than theatomic concentration of zinc, placing it in excess by a factor of two.The excess of magnesium in this illustrative example is not intended tochange the strengthening precipitate from MgZn₂ (though it may do so, asdetailed in the next paragraph), and as such, it does not significantlycontribute strength to the alloy. Nor does the excess of magnesium havea significant effect on the level of zinc accumulation at the interface,even though it can reduce or eliminate the level of free zinc in thematrix. This is because the interfacial accumulation of zinc duringanodizing occurs whether the zinc is in the matrix or bound in aprecipitate phase. In sufficient excess, however, the excess magnesiumdoes prevent delamination of the anodic oxide through thissulfur-scavenging mechanism. It may reduce the thermal and electricalconductivity of the alloy somewhat, but may be beneficial in terms ofcorrosion resistance.

Although, the excess of magnesium in the previous illustrative exampleis not intended to significantly contribute strength to an alloy wherethe strengthening phase was assumed to be MgZn₂, alternativestrengthening phases may be formed in some cases, and their roles mustthen be considered in determining precise alloying concentrations foroptimal strengthening under any given thermo-mechanical processingroute, and in turn for determining an appropriate excess of magnesium.This may allow equivalent strength to alloy 904 with lower zincconcentrations. Whatever the precise alloy composition selected foroptimal strengthening, an excess of magnesium can be preferred. A wideregion of interest for such candidate alloys exists, in the regionoutlined 908. Note that, in addition to custom compositions 904/906,region 908 encompasses lower zinc compositions, including those aboveand to the left of 904/906 on graph 900. A further benefit of the excessof magnesium will be discussed later.

It should be noted that the commercially available aluminum alloys shownin FIG. 9 include significant amounts of elements other than zinc andmagnesium, such as copper, iron, silicon and manganese, which can haveundesirable cosmetic effects. As described above, the presence ofcopper, manganese, and certain other elements in more thanmicro-alloying amounts can yellow a resultant anodic oxide tounacceptably high b* value levels. As described above, in someapplications the b* value of the anodized part, as viewed from anexposed surface of the anodic oxide (e.g., surface 701), should be lessthan 2, in some cases less than 1, or even less than 0.5. Thus, in someembodiments, custom alloy 904 and magnesium-rich custom alloy 906, havevery strictly controlled maxima for all other elements: e.g., 0.01weight % for magnesium, 0.01 weight % for chromium, 0.01 weight % forzirconium, 0.02 weight % for copper, 0.02 weight % for titanium, 0.05weight % for silicon, and a maximum of 0.01 weight % for any othernon-specified element, to a total maximum of 0.1 weight % fornon-specified others. Other elements may, however, be specifically addedas micro-alloying elements in accordance with the approach describedearlier in this paper.

It has been found that zinc and magnesium do not yellow a resultantanodic oxide. In fact, magnesium and/or zinc may tend to provide abluish high to the anodic oxide, which in certain applications is moredesirable than a yellow hue. Thus, magnesium-rich custom alloy 906 canprovide sulfur-scavenging capability as well as desired cosmetic (color)quality to an anodized part.

FIG. 10 shows flowchart 1000, illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate usinga sulfur-scavenging strategy. The aluminum alloy substrate can includezinc and magnesium alloys that give the substrate high tensile strength,with a balanced proportion of magnesium and zinc (e.g., atomic % zinc=2times atomic % magnesium to yield MgZn₂ η′ precipitates). At 1002, asulfur-scavenging species is incorporated into an aluminum alloysubstrate. In some embodiments, the sulfur-scavenging species is addedat a concentration ranging from about 0.5 weight % and about 3 weight %.In a particular embodiment, the sulfur-scavenging species is an excessof magnesium (i.e., a significant addition of magnesium, over and abovethe balanced level of half that of the atomic % zinc).

In some embodiments, the sulfur-scavenging species has a Gibbs freeenergy of oxide formation lower than that of aluminum. In someembodiments, the sulfur-scavenging species is additionally a strongsulfate former, i.e., has a large negative enthalpy for sulfateformation—in some embodiments, more negative than that for aluminumsulfate formation. In some embodiments, the sulfur-scavenging speciesincludes one or more of lithium, magnesium, calcium, strontium, andbarium. In particular embodiments, two or more types ofsulfur-scavenging species are used.

At 1004, the aluminum alloy substrate is anodized, using, for example,an anodic solution comprising sulfuric acid (e.g., type II anodizingprocess). During anodizing, the sulfur-scavenging species binds withsulfur species originating from the anodic solution. For example,lithium and/or magnesium and bind with sulfate ions to form lithiumsulfate and/or magnesium sulfate. In this way, the sulfur-scavengingspecies “scavenges” the sulfur species and prevents the sulfur speciesfrom binding with zinc to form a zinc-sulfur species, which is aninterface-weakening species, at an interface between the substrate and aresultant anodic oxide.

A further possible benefit of engineering an excess of magnesium, andthe correspondingly reduced level of free zinc in the matrix is that thedifferential growth rates of grains of different orientations (by virtueof zinc making the matrix more anodic in {111} orientations) may beeliminated. Thus, aluminum-zinc-magnesium alloys with an excess ofmagnesium are preferred for avoiding the afore-mentionedgrain-orientation related cosmetic defects. In a zinc-rich alloy, or analloy with a balanced ratio (such as Zn:2×Mg where MgZn₂ precipitatesare expected), anodic oxide forms on grains of {111} surface orientationapproximately 10% faster than on grains on other surface orientations(under typical Type II anodizing conditions at 1.5 A/dm²). When asubstantial excess of magnesium is employed to eliminate free zinc inthe matrix, this discrepancy is reduced to about 1-3% and the visualdefect is eliminated. That is, the magnesium can be added in excess overa balanced ratio for magnesium-zinc precipitate formation so as toeliminate or reduce a concentration of non-precipitated zinc in thealuminum alloy substrate in a T6 or T7 temper. This reduces adiscrepancy between growth rates of different portions of the anodicoxide on grains of distinct surface orientations, resulting in theanodic oxide having a thickness uniformity of within 5% among grains of{111} surface orientation and other surface orientations.

FIG. 12 shows how the addition of copper in micro-alloying amounts andlithium as a sulfur-scavenging species can improve adhesive of an anodicfilm to a substrate. In particular, FIG. 12 shows scanning electronmicroscope (SEM) images of three anodized substrate samples (1200, 1202,1204) after performing a 5-by-5 array of 10 kg Vickers indentationsspaced 350 micrometers apart, using an interfacial adhesion testingmethod as disclosed in U.S. patent application Ser. No. 14/678,881,which is disclosed herein in its entirety. In all samples 1202, 1204,and 1204, the anodic oxides are of 14 micrometer thickness, and wereformed using 1.5 A/dm² current density in 200 g/L sulfuric acidsolution.

Sample 1200 is an aluminum alloy substrate having 5.5 weight % of zincand 1.0 weight % of magnesium (corresponding to a balanced zinc andmagnesium aluminum alloy) without added micro-alloying element orsulfur-scavenging species. The SEM image of sample 1200 shows evidenceof significant anodic oxide detachment due to interface weakening byinterfacial enrichment of zinc, and its interaction with sulfur from theanodizing electrolyte. In particular, the back-scattered compositionalscanning electron microscope image of sample 1200 shows a number oflight areas corresponding to the bare aluminum substrate—where theapplied load has detached the anodic oxide. Some manufacturingrequirements require samples having less than 10 detachment regions tobe acceptable.

Sample 1202 is an aluminum alloy substrate having 5.5 weight % of zinc,1.0 weight % of magnesium, and 0.05 weight % copper as a micro-alloyingelement. As shown, the indentation test shows there are only four verymuch smaller light areas corresponding to interfacial adhesion failure.Thus, the addition of just a small amount of copper is sufficient toovercome the weak interfacial adhesion caused by interfacial enrichmentof zinc and zinc-sulfur species. Sample 1204 is an aluminum alloysubstrate having 5.5 weight % of zinc, 1.0 weight % of magnesium, and1.75 weight % lithium as a sulfur-scavenging element. The SEM image ofsample 1204 shows substantially no bright spots, thereby indicating theaddition of a sulfur-scavenging element (e.g., lithium) also overcomesthe delamination problem.

Combinations and Other Embodiments

In some cases, using a combination of a micro-alloying element and asulfur-scavenging species has been found to provide a combined benefit.For example, adding one or more micro-alloying elements and adding oneor more sulfur-scavenging species to an aluminum alloy composition canresult in an anodic oxide having an even higher resistance todelamination than the micro-alloying element or sulfur-scavengingspecies individually. For example, copper micro-alloying (which minimizezinc enrichment at the interface) may be used in combination withlithium and/or magnesium for their sulfur-scavenging ability.Alternatively, one or more sulfur-scavenging species can be added inlower amounts than would be used to prevent delamination alone, and oneor more micro-alloying elements can be added to make up for thedeficiency in sulfur-scavenging species, resulting in an anodic filmthat is resistant to delamination. This strategy can be used to addlesser amounts of elements that can cause yellow discoloration, such ascopper, iron, and/or silver.

FIG. 11 shows flowchart 1100, illustrating a process of increasing anadhesion strength of an anodic oxide to a high-strength substrate usinga combination of sulfur-scavenging species and micro-alloying element.At 1102, relative amounts of one or more sulfur-scavenging species andone or more micro-alloying elements required to achieve a pre-determineddelamination resistance of an anodic oxide on a high aluminum strengthsubstrate is determined. The pre-determined delamination resistance canbe a threshold value determined using, for example, thedelamination-resistance methods described in U.S. application Ser. No.14/678,881, which is incorporated herein in its entirety. Additionallyor alternatively, the relative amounts of a sulfur-scavenging speciesand a micro-alloying element can be determined by a pre-determineddiscoloration of the anodic oxide. For example, the pre-determineddiscoloration may be not be allowed to exceed a particular b* value(e.g., b*>1 or b*>0.5).

At 1104, the one or more sulfur-scavenging species and the one or moremicro-alloying element are incorporated into an aluminum alloysubstrate. At 1106, the aluminum alloy substrate is anodized such thatduring anodizing, the sulfur-scavenging species binds with sulfurspecies, and the micro-alloying element prevents at least some of thezinc from enriching at the interface between the substrate and anodicoxides. The combination of the sulfur-scavenging species and themicro-alloying element increases the adhesion strength of the anodicoxide and/or reduces the discoloration of the anodic oxide compared tousing a sulfur-scavenging species or a micro-alloying elementindividually.

In some cases, the addition of one or more sulfur-scavenging speciesand/or one or more micro-alloying elements can reduce the amount ofother alloying elements required to provide the high-tensile strength tothe aluminum alloy. For example, adding 0.05 weight % of coppermicro-alloying element has been found to reduce the amount of zincrequired for optimum strengthening by a corresponding 0.05 weight %,with the resultant aluminum alloy substrate having substantially thesame mechanical properties (e.g., yield strength of 340-350 MPa, andhardness of 125 HV), as the alloy having the full concentration of zinc.In a particular embodiment, this reduces the zinc composition to 5.45weight % compared to a nominal 5.5 weight % for the custom alloy 904 ofFIG. 9. This can be some benefit since aluminum alloys having higherzinc composition can present corrosion problems while lower zinccompositions (without the micro-alloying element) can detrimentallyaffect the strength of the alloy substrate. Once the amount of zinc isfixed, a corresponding amount of magnesium to provide a balanced alloycan be determined. In some embodiments, the magnesium content issubstantially increased over this balanced amount to provide thesulfur-scavenging benefits described above or to minimize free zinc inthe matrix, so as to reduce grain-orientation related defects. In thisway, customized anodized alloys having prescribed tensile strength,anodic oxide delamination resistance, and/or color can be designed.

It should be noted that embodiments presented herein can be used incombination with one or more embodiments described in related U.S.application Ser. No. 14/474,021, 14/593,845, 14/678,881, and 14/678,868,which are incorporated herein in their entireties. For example, theembodiments described herein may be used in combination with apost-anodizing heat treatment to diffuse zinc away from the interface,either for greater robustness, or to allow for shorter or lowertemperature heat treatments to achieve the same effect.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of the specificembodiments described herein are presented for purposes of illustrationand description. They are not targeted to be exhaustive or to limit theembodiments to the precise forms disclosed. It will be apparent to oneof ordinary skill in the art that many modifications and variations arepossible in view of the above teachings.

What is claimed is:
 1. An enclosure for an electronic device, theenclosure comprising: an aluminum alloy substrate including zinc,magnesium, and a micro-alloying element, wherein the micro-alloyingelement is added to a target concentration, wherein the targetconcentration is no more than 0.1 weight %; and an anodic oxide formedon the aluminum alloy substrate, wherein the micro-alloying element isenriched at an interface between the aluminum alloy substrate and theanodic oxide.
 2. The enclosure of claim 1, wherein the aluminum alloysubstrate includes additional elements other than zinc, magnesium, andthe micro-alloying element, wherein the additional elements comprise:chromium at no more than 0.01 weight % concentration, copper at no morethan 0.01 weight % concentration, manganese at no more than 0.01 weight% concentration, zirconium at no more than 0.01 weight % concentration,titanium at no more than 0.02 weight % concentration, silicon at no morethan 0.05 weight % concentration, iron at no more than 0.08 weight %concentration, and any other element at no more than 0.01 weight %concentration, to a total maximum of 0.1 weight % concentration of theadditional elements.
 3. The enclosure of claim 1, wherein the anodicoxide has a thicknesses of at least 10 micrometers, wherein a color ofthe anodic oxide in a non-dyed state is within 1 b* value, andoptionally within 0.5 b* value, based on D65 white spot colormeasurement convention.
 4. The enclosure of claim 1, wherein thealuminum alloy substrate has substantially balanced concentrations ofzinc and magnesium for achieving an optimal strength throughprecipitation upon ageing.
 5. The enclosure of claim 4, wherein thealuminum alloy substrate comprises an atomic % of zinc that is aboutdouble the atomic % of magnesium so as to form MgZn₂ precipitates. 6.The enclosure of claim 4, wherein the aluminum alloy substrate comprisesa zinc concentration of about 5.5 weight % and a magnesium concentrationof about 1 weight %.
 7. The enclosure of claim 1, wherein themicro-alloying element has a higher Gibbs free energy of oxide formationthan the zinc.
 8. The enclosure of claim 1, wherein the micro-alloyingelement is selected from the group consisting of vanadium, germanium,cobalt, antimony, copper, tellurium, osmium, selenium, iridium, rhodium,palladium, silver, and gold.
 9. The enclosure of claim 1, wherein themicro-alloying element is selected from the group consisting of copper,silver, and antimony.
 10. The enclosure of claim 1, wherein the anodicoxide is grown to a thickness of at least about 10 micrometers using aType II anodizing process, wherein the aluminum alloy substrate with theanodic oxide has a b* value of less than 1 based on CIE 1976 L*a*b*color measurement convention.
 11. The enclosure of claim 1, wherein themicro-alloying element includes more than one type of element.
 12. Theenclosure of claim 11, wherein a first type of element reducesenrichment of the zinc at the interface and a second type of elementoffsets discoloration of the anodic oxide due to the presence of thefirst type of element.
 13. A method of forming an enclosure for anelectronic device, the method comprising: anodizing an aluminum alloysubstrate comprising zinc, magnesium, and a micro-alloying element,wherein the micro-alloying element is added to a target concentration,wherein the target concentration is no more than 0.1 weight %, whereinthe micro-alloying element reduces enrichment of the zinc at aninterface between the aluminum alloy substrate and a resultant anodicoxide, enrichment of the zinc at the interface associated with reducingan adhesion of the anodic oxide to the aluminum alloy substrate.
 14. Themethod of claim 13, wherein an adhesion strength of the resultant anodicoxide as measured by 5-by-5 array 10 kg Vickers indentations spaced 350micrometers apart and as viewed by scanning electron microscope imagingis less than 10 detached regions of the anodic oxide.
 15. The method ofclaim 13, wherein the aluminum alloy substrate includes additionalelements other than zinc, magnesium, and the micro-alloying element,wherein the additional elements comprise: chromium at no more than 0.01weight % concentration, copper at no more than 0.01 weight %concentration, manganese at no more than 0.01 weight % concentration,zirconium at no more than 0.01 weight % concentration, titanium at nomore than 0.02 weight % concentration, silicon at no more than 0.05weight % concentration, iron at no more than 0.08 weight %concentration, and any other element at no more than 0.01 weight %concentration, to a total maximum of 0.1 weight % concentration of theadditional elements.
 16. The method of claim 13, wherein the anodicoxide is grown to a thicknesses of at least 10 micrometers, wherein acolor of the anodic oxide in a non-dyed state is within 1 b* value, andoptionally within 0.5 b* value, based on D65 white spot colormeasurement convention.
 17. The method of claim 13, wherein the aluminumalloy substrate has substantially balanced concentrations of zinc andmagnesium for achieving an optimal strength through precipitation uponageing.
 18. A method of forming an enclosure for an electronic device,the method comprising: anodizing an aluminum alloy substrate comprisingzinc, magnesium, and a micro-alloying element, wherein themicro-alloying element is added to a target concentration, wherein thetarget concentration is no more than 0.1 weight %, wherein themicro-alloying element reduces a discrepancy between growth rates ofdifferent portions of an anodic oxide on grains of distinct surfaceorientations, resulting in an anodic oxide having a thickness uniformityof within 5% between grains of {111} surface orientation and othersurface orientations.
 19. The method of claim 18, wherein the aluminumalloy substrate includes additional elements other than zinc, magnesium,and the micro-alloying element, wherein the additional elementscomprise: chromium at no more than 0.01 weight % concentration, copperat no more than 0.01 weight % concentration, manganese at no more than0.01 weight % concentration, zirconium at no more than 0.01 weight %concentration, titanium at no more than 0.02 weight % concentration,silicon at no more than 0.05 weight % concentration, iron at no morethan 0.08 weight % concentration, and any other element at no more than0.01 weight % concentration, to a total maximum of 0.1 weight %concentration of the additional elements.
 20. The method of claim 16,wherein the micro-alloying element is selected from the group consistingof vanadium, germanium, cobalt, antimony, copper, tellurium, osmium,selenium, iridium, rhodium, palladium, silver, and gold.